The Role of ETV6/TEL in Hematopoiesis and Lymphoid Neoplasia.

Abstract 972

The transcriptional repressor TEL (Translocation Ets Leukemia) also known as ETV6 (Ets variant gene 6) located on human chromosome 12 is essential for hematopoietic stem cell (HSC) maintenance. Abnormalities involving TEL on the short arm of chromosome 12 including deletions and translocations are common in a surprisingly wide spectrum of hematological and non-hematological malignancies. Genome-wide analysis of leukemic samples revealed the loss of TEL expression and loss of heterozygousity (LOH) in more than half of B-cell ALL patients. We have previously reported on the generation of transgenic zebrafish expressing TEL-AML1 that develop pre-B ALL after long latency, and recapitulate the same phenotype and molecular events in human leukemias. In this study, we first used these transgenic fish to investigate the role of TEL in leukemia development. Our data shows that the loss of TEL expression contributes to, and accelerates the leukemic phenotype in our TEL-AML1 transgenic zebrafish. TEL is a selective regulator of lymphoid and HSC survival. The mechanism by which TEL modulates adult lymphoid and HSC renewal is yet to be elucidated. Moreover, studies of TEL expression and role in lymphoma development are lacking. At the genetic level, the highly conserved alternative start codon, and the initiation of translation at Methionine-43 of TEL creates a shorter isoform that is mostly nuclear. Utilizing specific antibodies that distinguish between the expression patterns of the two TEL isoforms, we demonstrate that the differential cytoplasmic and nuclear localization of TEL isoforms in human cells is involved in B- and T-cell lymphoid maturation. Our studies uncover a nuclear trafficking mechanism that is likely to regulate the nuclear repression and tumor suppressor functions of TEL. We next investigated the role of TEL in normal hematopoiesis, and TEL tumor suppressor functions in lymphoid malignancies in a novel model of zebrafish lymphoma. To study the role of TEL in hematopoiesis, we isolated the zebrafish TEL coding region and promoter elements from a BAC library, and found that TEL is expressed in early embryonic stages, in hematopoietic and neural lineages, and its expression colocalized with the hematopoietic stem cell (HSC) markers cMyb, Flk1 and RUNX1. In this model, the loss of TEL expression using morpholinos and dominant negative approaches in wild type zebrafish led to the development of lymphomas in 15% of these fish. This prompted us to investigate the expression of TEL in human lymphomas. Loss of TEL expression or the cytoplasmic localization of TEL isoforms in tumor cells from lymphoma patients were detected in eleven out of twenty three patients investigated so far (48%), and correlated with the non-Hodgkin diffuse large B-cell lymphomas and follicular subtypes as well as the aggressiveness of lymphomas. Flow cytometric analyses of lymphoma samples associated with loss of TEL expression demonstrated a CD19^+ve, CD20^+ve, CD10^+ve, and CD45^+ve phenotype. In cellular assays, we demonstrate that the activity and cellular localization of TEL are tightly regulated at multiple levels, and coordinated by P38 kinase activity. Our studies highlight a novel tumor suppressor role of TEL/ETV6 in lymphoma development, and establish TEL/ETV6 expression as a useful biomarker for non-Hodgkin lymphoma diagnosis and disease progression.